Most hydraulic or pneumatic systems employ elastomeric and/or plastic sealing rings to prevent the flow of pressurized fluid past the system's piston member. Such sealing rings are typically held into sealing compression against the interior of the system's piston cylinder by a loader support member. The loader is seated in a radial groove around the piston such that it provides a loading force to seal any gaps between the rod surface and the surface of the interior of the cylinder.
Piston systems that are subjected to high temperatures experience thermal expansion, increasing friction between the sealing ring and the interior of the cylinder. Thermal expansion and high temperatures therefore increase the risk of seal extrusion and seal damage.
Alternately, or in addition to thermal expansion, high fluid pressures increase seal friction and risk of seal extrusion and seal damage. In either case, friction and pressure each hinder smooth axial movement and may cause the seal to extrude or roll in the gap between the piston and cylinder bore, increasing the opportunity for fluid leakage/seal failure. Thus, in high pressure and/or temperature applications, it is advantageous to provide a robust seal that prevents extrusion past the piston head and ultimately prevents damage to the sealing ring or loader.
Damage to the sealing ring or loader frequently presents itself as “nibbling”, which is simply to say that fragments of the ring/loader are tugged off. This not only damages the ring/loader, which can no longer offer as effective a seal, but also the system as a whole, introducing chunks of material from the nibbled areas that contaminate the system, which can cause even further damage.
A simple and inexpensive way to offer a seal with a higher pressure rating and improved extrusion resistance is to provide rigid or semi-rigid anti-extrusion rings, or back-up rings, adjacent to one or both sides of the sealing ring. Anti-extrusion rings support sealing rings and/or loaders from extruding between the piston and the bore. Further, anti-extrusion rings themselves may present a barrier against extrusion, nibbling, and/or rolling of the loader and/or seal ring.
Installation of a continuous (unbroken) anti-extrusion ring onto a piston would require removal of the sealing ring and possibly the loader or require a split piston design, depending on the configuration of the piston system. In addition, anti-extrusion rings are generally rigid, making it difficult to s retch over the piston. Thus, for the easiest installation of anti-extrusion rings, they are commonly split across their circumference. However, once cut, anti-extrusion rings tend to separate or overlap at the split, particularly under the circumstances under which they would be most likely to be used, i.e., high pressure and/or high or low temperature situations. Such separation or overlap at the split can prevent the anti-extrusion rings from performing effectively in support of the sealing ring and/or loader and may in and of itself cause damage to the system.